Apparatus and Method of Image Registration

ABSTRACT

A method for bringing an IVUS and an OCT image into register. In one embodiment, the method includes obtaining an IVUS image of an area of a lumen; obtaining an OCT image of the same area of the lumen; determining the same asymmetry in each of the IVUS and OCT images; and overlaying the IVUS and OCT images and rotating them with respect to one another until the asymmetry in each of the IVUS and OCT images are in register, and determining the angle of rotation that resulted in the registration. In another aspect, the invention relates to a probe for OCT and IVUS imaging. In one embodiment, the probe includes a sheath having a first end and a second end defining a lumen; a marker that is opaque to light and ultrasound located between the first end and second end; and an IVUS/OCT probe head positioned within the sheath.

FIELD OF THE INVENTION

The invention relates generally to the field of medical imaging and morespecifically to the field of Optical Coherent Tomography (OCT) andIntraVascular UltraSound (IVUS) imaging.

BACKGROUND OF THE INVENTION

OCT interferometric methods deliver light onto a sample of interest,such as the wall of a lumen of a blood vessel, and collect a portion ofthe light returned from the sample. Due to the size and complexity ofmany light sources and light analysis devices, the sources and lightdetectors are typically located remotely from the sample area ofinterest. One method of optically analyzing internal parts is to guidelight from a remote light source onto the sample using a thin opticalfiber that is minimally disruptive to the normal function of the sample.This minimal disruption occurs because of the diminutive cross-sectionof the optical fiber.

There are many miniature optical systems known in the art that can beused for analysis of internal lumen structures. Each optical system canbe conceptually divided into a beam delivery and focusing means, and abeam directing means. Light is passed from an external light source tothe internal lumen through one or more optical illumination fibers,which may be single mode or multimode in nature. The illumination fiberis in communication with the miniature optical system, which focuses anddirects the beam into the luminal wall.

Light is reflected by the lumen wall and transmitted to an analysisapparatus outside the body, generally using the same fiber astransmitted the incident light. The analysis apparatus is typicallyinterferometric and the resulting interferometric patterns are detectedand transformed into an image by a computer. In use, the fiber spinswithin the lumen of the vessel, thereby sweeping the wall with the lightand collecting the reflected light. Each revolution of the fibertherefore produces a scanned cross-sectional image of the vessel. As thefiber is retracted or pulled out of the vessel, a cylindrical image of aportion of the vessel lumen is obtained.

An IVUS probe is similar to an OCT probe; however, the IVUS probe usesultrasound rather than light. Ultrasonic pulses are produced by an IVUStransducer at the probe tip and the sound reflected by the walls of thelumen is received by the transducer and converted into electricalsignals which are then analyzed by a computer. As with OCT imaging, theIVUS probe spins in the lumen of the vessel and this results in areflection pattern from the walls of the vessel that is also analyzableby computer to produce a cross-sectional image. Again, as the IVUS probeis withdrawn from the vessel, a cylindrical image of a portion of thevessel lumen is obtained.

Now that combination OCT and IVUS probes have become available, it ispossible to acquire both images in one procedure. Obtaining both of theIVUS and OCT images of the vessel has benefits, because of the inherentadvantages and limitations of each imaging modality. OCT light tends tobe scattered by particles, such as blood cells passing through the lumenof the vessel. This scattering degrades the resulting OCT image of thevessel wall. However, OCT light penetrates into the wall of the vesselthereby being able not only to image the wall of the vessel but also theintima of the vessel. IVUS ultrasonic pulses are not as affected byparticles in the lumen. Therefore, IVUS and OCT images can complementone another.

Because the two images, whether taken with the same probe or differentprobes, have different resolution, the alignment of the images to form ausable composite image is difficult. In addition, because of otherlimitations that will be discussed below, accurate determination ofactual distances in the images is difficult. What is needed is a way toprocess the OCT and IVUS images so that both images depict the sameregion of the vessel, at the same magnification and orientation, andwith a mechanism to check the calibration of each imaging modality.

The present invention addresses this need.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a probe for OCT and IVUSimaging. In one embodiment, the probe includes a sheath having a firstend and a second end, and a wall defining a lumen; a marker that isopaque to light and ultrasound and is located between the first end andsecond end of the sheath; and an IVUS/OCT probe head positioned withinthe sheath. In another embodiment, the marker is a cylindrical annulardevice positioned on the outside wall of the sheath. In yet anotherembodiment, the marker is a cylindrical annular device positioned on theinside wall of the sheath. In still another embodiment, the marker is acylindrical annular device positioned within the wall of the sheath. Instill yet another embodiment, the marker is a bar positioned on theoutside wall of the sheath. In yet another embodiment, the marker is abar positioned on the inside wall of the sheath. In still yet anotherembodiment, the marker is a bar positioned within the wall of thesheath.

In another embodiment, the system includes a probe for OCT and IVUSimaging that includes a sheath having a first end and a second end and awall defining a lumen; a marker that is opaque to light and ultrasoundand is located between the first end and second end of the sheath; andan IVUS/OCT probe head positioned within the sheath; and an analysis andinstrumentation unit in communication with the probe; and a display incommunication with the analysis and instrumentation unit. In anotherembodiment, the processor of the analysis and instrumentation unit isconfigured to locate the marker in each of an OCT image and an IVUSimage, and rotates at least one of the OCT image and IVUS image untilthe marker in both images is in register. In yet another embodiment, thesystem includes a probe for OCT and IVUS imaging; an analysis andinstrumentation unit in communication with the probe; and a display incommunication with the analysis and instrumentation unit. In still yetanother embodiment, the processor is configured to locate the samesymmetry in each of an OCT image and an IVUS image, and rotate at leastone of the OCT image and IVUS image until the asymmetry in both imagesis in register.

In one embodiment, the asymmetry in the OCT image and the IVUS image iscaused by a guidewire. In another embodiment, the asymmetry in the OCTimage and the IVUS image is caused by the shape of the lumen beingimaged. In yet another embodiment, the asymmetry in the OCT image andthe IVUS image is caused by the internal morphology of the lumen.

In another aspect, the invention relates to a method of bringing an IVUSimage and an OCT image into register. In one embodiment, the methodincludes obtaining an IVUS image of an area of a lumen; obtaining an OCTimage of the same area of the lumen; determining the same asymmetry ineach of the IVUS and OCT images; and overlaying the IVUS and OCT imagesand rotating them with respect to one another until the asymmetry ineach of the IVUS and OCT images are in register, and determining theangle of rotation that resulted in the registration. In anotherembodiment, the asymmetry in the OCT image and the IVUS image is causedby a guidewire. In yet another embodiment, the asymmetry in the OCTimage and the IVUS image is caused by the shape of the lumen beingimaged. In still yet another embodiment, the asymmetry in the OCT imageand the IVUS image is caused by the internal morphology of the lumen. Inone embodiment, the method includes obtaining an IVUS image of an areaof a lumen having a marker; obtaining an OCT image of the same area ofthe lumen having the marker; determining the orientation of the markerin each of the IVUS and OCT images; and overlaying the IVUS and OCTimages and rotating them with respect to one another until the marker ineach of the IVUS and OCT images are in register, and determining theangle of rotation that resulted in the registration.

In another aspect, the invention relates to a method of checking thecalibration of an OCT system. In one embodiment, the system includes aprobe having a probe head in a sheath having a reflector. In anotherembodiment, the method uses an IVUS system having an ultrasonictransducer on the probe head. In yet another embodiment, the methodincludes the steps of measuring the amount of time it takes for theultrasonic pulse to leave and return to the IVUS transducer; calibratingthe OCT system by measuring the distance to the known reflector in thesheath; acquiring an IVUS image and an OCT image of a vessel comprisinga wall defining a lumen; processing each image to measure the distancefrom the probe location in the lumen to the wall in the vessel; andcomparing the OCT distance measurement with the IVUS distancemeasurement to determine if the two measurements are equivalent towithin a predetermined value.

Another aspect of the invention relates to a method for determining theidentity of the fluid being used as a flushing fluid in a vessel havinga wall having a landmark, the wall defining a lumen. In one embodiment,the method includes the steps of measuring the distance to a landmark onthe wall of the vessel from a probe head using OCT; measuring theround-trip time it takes for the IVUS ultrasonic pulse to reach the samelandmark and return; dividing the OCT measured distance by half the IVUSmeasured round-trip time to obtain a measured speed of sound in thefluid; and comparing the measured speed of sound in the fluid to thespeed of sound in various flushing solutions.

Another aspect of the invention relates to a method of characterizing atissue. In one embodiment, the method includes the steps of measuring,using OCT, the distance to an edge of the tissue; measuring, using OCT,the distance to another landmark within the tissue; subtracting thedistance to the edge of the tissue from the distance to the landmark inthe tissue to determine a thickness of the tissue between the tissueedge and the landmark; measuring the time it takes for an IVUS acousticpulse to reach the same edge of the tissue; measuring the time it takesfor the IVUS acoustic pulse to reach the landmark; subtracting the timeit takes the IVUS acoustic pulse to reach the edge of the tissue fromthe time it takes the IVUS acoustic pulse to reach landmark to calculatethe transit time through the tissue to the landmark; dividing thethickness of the tissue measured by OCT by the transit time measured byIVUS, to determine the measured speed of sound in the tissue; andcomparing the measured speed of sound in the tissue to the speed ofsound in various tissues to determine the tissue characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and function of the invention can be best understood fromthe description herein in conjunction with the accompanying figures. Thefigures are not necessarily to scale, emphasis instead generally beingplaced upon illustrative principles. The figures are to be consideredillustrative in all aspects and are not intended to limit the invention,the scope of which is defined only by the claims.

FIG. 1 is a block diagram of an embodiment of an OCT/IVUS system;

FIG. 2(a) is a schematic diagram of an embodiment of the IVUS and OCThead of the probe of FIG. 1;

FIG. 2(b) is a diagram of the angular divergence of the ultrasonic andlight beams of the probe head of FIG. 2(a);

FIG. 3 is a highly schematic diagram of an embodiment of the ultrasonicand light beams of the probe head of FIG. 2(a) sweeping out a volume ofspace;

FIG. 4(a) is a diagram of an embodiment of a probe with a discontinuousfiducial marker and a bar fiducial marker;

FIG. 4(b) is an end view of the discontinuous fiducial marker of FIG.4(a);

FIG. 4(c) is an end view of the bar fiducial marker of FIG. 4(a);

FIG. 4(d) is an image view of the discontinuous fiducial marker of FIG.4(a) taken by IVUS and OCT;

FIG. 5 is an OCT image of a vessel with a guidewire shadow;

FIG. 6 is a flow diagram of an embodiment of a method for rotationalalignment;

FIG. 7 is a flow diagram of an embodiment of a method for radialcalibration and alignment;

FIG. 8 is a flow diagram of another embodiment of a method for radialcalibration and alignment; and

FIG. 9 is a diagram of an embodiment of the components of measurement oftissue characteristics using IVUS and OCT.

DETAILED DESCRIPTION

The issue of calibration and alignment of an IVUS and an OCT imageoccurs whether the IVUS and OCT components are located on the same probeor different probes. Even if the IVUS and OCT transducers are in thesame probe, actual manufacturing tolerances permit sufficient deviationin the direction of the beam emitting from each of the OCT and IVUScomponents that the beams are not parallel.

Referring to FIG. 1, a combination IVUS and OCT system 10 is shown inblock diagram. The system 10 includes an analysis and instrumentationunit (AIU) 12, which includes a processor and an interferometer for usewith the OCT optics of the probe, and an ultrasound generator for usewith the IVUS transducer of the probe. The analysis and instrumentationunit 12 is in communication with a display 15. The analysis andinstrumentation unit 12 is connected by electrical conductors andoptical fibers 17 to a patient interface unit (PIU) 20. The PIU 20includes the motors, optical and electrical connections necessary torotate, translate, and provide current and light to the probe 25. Theprobe 25 is removably coupled to the PIU 20 by way of a removableelectrical/optical coupler 26. The probe itself includes an opticalfiber/electrical conductor combination 28 within a sheath 31. Theoptical fiber/electrical conductor combination 28 connects to the OCToptics 33 and the IVUS transducer 34.

In more detail and referring to FIG. 2(a), a view of the OCT/IVUS head50 of the probe 25 includes an ultrasonic transducer 34 and an opticalbeam director 52 that is part of the optical train (lenses, collimator,etc) 60 of the OCT optics. As stated above, the electrical conductors 64and optical fiber 68 form the optical fiber/electrical conductorcombination 28.

The ultrasonic transducer 34 generates an ultrasonic beam 80perpendicular to the surface of the transducer 34. Similarly, the beamdirector 52 directs light along a beam 84 substantially parallel to theultrasonic beam 80. The two beams, ultrasonic and light should bedirected in parallel but due to manufacturing limitations the maydeviate by a few degrees (θ). The result is that the IVUS image of thevessel and the OCT image of the vessel may be rotated (θ) degrees withrespect to one another (FIG. 2B).

Referring to FIG. 3, as the head of the probe 50 spins and is withdrawnfrom the vessel, the path 90 of the optical beam 84 and ultrasound beam80 against the vessel wall is in the form of a spiral. In the embodimentshown, the IVUS transducer 34 is further forward in the probe head thanthe beam director 54, so that as the probe head 50 is withdrawn from thevessel (directional arrow P), the OCT beam 84 images an area of thevessel (S_(OCT)) several revolutions of the head 50 earlier than theultrasound beam 80 images substantially the same the area (S_(US)).Thus, to image the same portion of the vessel wall, the software of thesystem must take into account the delay caused by the difference inplacement of the IVUS transducer 34 and the beam director 54 in the headof the probe 50. However, even when this delay is accounted for, theIVUS image and the OCT image may be rotated with respect to one anotheras described above.

In one embodiment, one way to address this radial offset (θ) is tomeasure the offset accurately after manufacture. This may beaccomplished by placing the probe head into a cylindrical fixture havingknown patterns on the walls of its lumen. Next the probe head iswithdrawn and IVUS and OCT images are captured. Because the actualfeatures are known, the images taken with OCT and IVUS can then beco-registered. By counting the number of frames between the OCT image ofan area of wall and the IVUS image of the same area of wall, the delaybetween images can be determined. By noting how many degrees of rotationare needed to align the images, (θ) can be determined. This informationmay be encoded into a bar code or equivalent or an RFID tag attached tothe probe connector 20 (FIG. 1). The user of the system 10 can thenprogram this information into the system using a bar code reader or RFIDreader so that the IVUS and OCT images are displayed together with thesame orientation and magnification.

Alternatively, the system can self-calibrate using one of severalimaging techniques. In one embodiment, referring to FIG. 4(a), thesheath 31 (FIG. 1) is manufactured with a discontinuous cylindricalannulus 100 attached to the outside wall or inside wall of the sheath31. In FIG. 4(b), an end-on view of the annulus 100′ is shown.Alternatively, the sheath may have a fiducial bar 104, shown end-on inFIG. 4(c). In both cases, the fiducial bar 104 and discontinuouscylindrical annulus 100, collectively referred to as a fiducial marker,provide an asymmetry in the image which is detectable by both OCT andIVUS imaging. Note that the fiducial bar 104 and the discontinuousfiducial annulus 100 are not used together on the same sheath 31, andare shown together only to provide positioning information. The fiducialmarker may be made of any optically opaque material that has a differentoptical density and acoustic attenuation than the sheath 31. Thefiducial marker may be positioned anywhere along the sheath 31, butgenerally near one end of the region through which the probe head mustpass during pull back.

The ends of the pull-back are typically used to position the fiducialmarker in order to make sure the imaging of the regions of interest inthe vessel wall are not blocked by the fiducial marker. FIG. 4(d) showsan image 108 of the marker 100 as seen by one modality, and the sameimage as seen by the other modality 112. The difference in orientationbetween the split in the discontinuous annulus 116 taken by onemodality, such as IVUS, and the image of the split taken by the othermodality (112), such as OCT, is the angle (O). The system 10 can thendetermine how much to rotate the images 112, 116 so as to have the IVUSand OCT images coincide.

Another embodiment by which the system 10 can self-calibrate is byidentifying asymmetries in the images recorded by both the IVUS and theOCT modalities. One asymmetry that may be used is caused by a guidewirewhich the probe follows to its location in the blood vessel. Referringto FIG. 5, in this OCT image of a blood vessel, the wall of the vessel120 appears as a light boundary surrounding the black lumen 122 of thevessel. The guide wire passes through the lumen and is opaque to thelight from the OCT probe. As a result, the guidewire appears to be abreak or shadow 124 across the lumen boundary wall 120.

In one embodiment, the same software that determines if this form ofshadow is a guidewire or a vessel branch in general OCT imagingapplications is used to determine the orientation of the image byfinding the center point of the shadow. A discontinuity in the image ofthe vessel wall caused by the guidewire also appears in an IVUS imagebecause the guidewire is opaque to the ultrasonic beam. Thus, by usingthe discontinuity caused by the guidewire, one can again rotate theimages until the guidewire discontinuities are aligned in bothmodalities.

In another embodiment, the same procedure can be used with asymmetriesin the vessel wall itself. For example, an image of a vesselcross-section may show that the vessel is not circular but is some othershape, such as ellipsoidal, that is not a 360° symmetrical shape. Inthis case, both OCT and IVUS image generating programs are capable offinding the major and minor axes of the cross-section. Once these axesare found, one image is rotated until the major and minor axes of theimages of both modalities are in register. This technique can also bedone using some other asymmetry in the vessel cross-sectional image, forexample the presence of plaque. However, identifying a specific plaquelesion by one or the other modality may be difficult.

In more detail, in one embodiment, the OCT and IVUS images are alignedby rotationally transforming one helical image set relative to the otherbased upon the time delay between the ultrasound and optical imagingbeams. That is, given that OCT systems and ultrasound systems havedifferent resolution ranges and depths of penetration, it is desirableto display a fused image or overlaid image of the lumen that includesboth OCT data and ultrasound data. To do this with an axially displacedpair of acoustic and optical sensors in the probe tip requires arotational transformation of the acquired data sets so that images ofthe same anatomic locations (acquired at offset times) are registeredover each other as discussed above. Once the images are rotationallytransformed, the two images can be fused together. A number of differenttechniques can be used to combine portions of images, or otherwisegenerate new images, from some or all of an IVUS image and an OCT imagegenerated using one of the embodiments described herein.

Referring to FIG. 6, a generalized flow chart of the method forrotationally aligning two images is shown. The two images to be alignedare acquired 150, and landmarks, as discussed above, are identified 154.The magnification for the two images is equalized. The images are thenrotated until the distance between the same point of the landmark inboth images is minimized 158.

In addition to rotational registration, the combination of IVUS and OCTprovides additional data that is useful in determining magnification,confirming calibration, confirming the nature of the flush solution, anddetermining tissue characteristics.

Distance measurements using OCT are difficult. Generally, thisdifficulty arises because the measurement of distance using interferencefringes is affected by the relative lengths of the measurement arm andthe reference arm of the interferometer. The measurement arm of theinterferometer includes the length of the fiber in the probe itself, andthe length of the fiber connecting the probe to the interferometer. Thismeans that small distance measurements within a vessel are a very smallpart in what is a very long measurement arm. The fiber in the probe issubstantially at the temperature of the fluid used to flush blood fromthe vessel, while portions of the rest of the fiber measurement arm thatare not in contact with the flush fluid may be at a differenttemperatures. Because these temperatures may vary over time, the lengthsof the various fiber portions exposed to different changing temperatureswill change differentially from one another, causing the interferencefringes to shift. The resulting error in the measurement of distance inthe lumen is therefore large.

One attempt to address this error is to put a known reflector in thelight path at an approximately known distance from the probe head. Thisreflector is generally placed on the sheath that surrounds the opticalfiber. This reflector provides an approximate distance value that maythen be used to calibrate distances in the OCT image. However, becausethe head position within the sheath and within the lumen moves as thehead rotates, and because of the temperature variations, a second methodof checking the calibration of the OCT system is desired. The IVUSportion of the probe is used for this purpose.

Because the time it takes for an ultrasonic pulse to move from thetransducer and return to the transducer after reflecting from the wallof the vessel can be measured very accurately, and because the speed ofsound in the flush solution is known, the distance to the surface of thewall is accurately known. Therefore, by comparing the distance to thewall as measured by OCT and the distance to the wall as measured byIVUS, the accuracy of the calibration of the OCT portion of the devicemay be determined.

Referring to FIG. 7, to determine if the calibration of the OCT systemis accurate, one first calibrates the IVUS system by measuring theamount of time it takes for the ultrasonic pulse to leave and return tothe transducer 170 and calibrates the OCT system by measuring thedistance to a known reflector in the sheath 174. Next, an IVUS image 178and an OCT image 182 of the vessel are acquired. Each image is processed(186 for IVUS and 190 for OCT) to measure the distance from the centerprobe location in the lumen to the wall in the vessel. The two distancemeasurements (OCT and IVUS) are compared 194 to determine if they areequivalent to within a predetermined value. If they are, the calibrationis deemed accurate 198 and their relative magnifications are known. Ifthey are not, the systems must be recalibrated 202.

Referring to FIG. 8, another embodiment of a method to calibrate an IVUSand an OCT system is to calibrate 210 one of the modalities IVUS or OCT(typically the IVUS portion) and then acquire an image of the vesselwith that modality 214. An image is acquired with the other modality 218and each image is processed 222, 226 to measure the distance from thecenter of the probe location in the lumen to a landmark, such as a wall.The two distance measurements are compared 230 to determine if they areequivalent to within a predetermined value and if they are not, theuncalibrated modality is adjusted until the lengths are equivalent.

In addition to calibration, the combined modalities are useful inconfirming the identity of the fluid being used as a flushing fluid. Todo this, the distance to a landmark on the wall of the vessel ismeasured using OCT. The time it takes for the IVUS ultrasonic pulse toreach the same landmark and return is also measured. By dividing the OCTmeasured distance by half the IVUS measured round-trip time, the speedof sound in the fluid is calculated. By looking up the speed of sound inthe various possible flushing solutions, one can determine whichsolution is being used. If the solution as determined by the method isdifferent from the solution name entered into the system by theclinician during the setup of the imaging procedure, an alarm may begiven that the wrong fluid is being used and hence the measurements maynot be correct.

Finally, referring to FIG. 9, a probe head 50 is shown within a sheath31 within the lumen 122 of a vessel 250. A lesion 254 on the wall 250 issubjected to the OCT light beam 258 and the IVUS acoustic beam 262. TheOCT beam 258 measures the distance to the surface 264 of the lesion 254and the distance to another landmark 268 inside the lesion 254. Fromthese two measurements, the thickness of the lesion 254 between itssurface 264 and the landmark 268 is determined. The time it takes forthe IVUS acoustic pulse to reach the same surface 264 of the lesion andthe time it takes for the acoustic pulse to reach the landmark 268 isalso measured. By subtracting the two times, the transit time throughthe lesion is calculated. By dividing the thickness of the lesionmeasured by OCT by one half of the transit time, measured by IVUS, thespeed of sound in the lesion is determined. Because different lesionsare characterized by different speeds of sound, the type of lesion canthen be determined.

In one embodiment, the OCT and IVUS image data can be combined bydisplaying OCT data for a specific depth of the image and then limitingthe rest of the data displayed after that depth to IVUS data. Thisapproach can be modified such that the OCT data is displayed until thedepth where the OCT signal can no longer be discerned above the noisefloor is reached, and then the remainder of the image is filled in withthe deeper penetrating IVUS image data.

More sophisticated image data combination is also possible if eachsample is a weighted combination of the IVUS and OCT grayscale adjustedfor sample depth, and grayscale of adjacent samples. In one embodiment,each of the IVUS data set and the OCT data set includes a respectivegrayscale for each sample and depth information for each sample. In oneembodiment, the method includes the step of generating a combinedgrayscale for each sample in an image generated using the OCT data setand the IVUS data. The combined grayscale is based on the grayscale ofthe IVUS dataset and the grayscale of the OCT dataset in the sample andthe surrounding samples, as well as the depth of that sample.Additionally, identification of the tissue type may be possible bycontrasting the differential absorption of the IVUS and OCT energy by asample. That is, tissue that is reflective in one domain (OCT) may betransmissive in the other domain (IVUS). Thus, it is possible tohighlight a region as a calcium plaque or lipid plaque based ondifferences in transmission.

The aspects, embodiments, features, and examples of the invention are tobe considered illustrative in all respects and are not intended to limitthe invention, the scope of which is defined only by the claims. Otherembodiments, modifications, and usages will be apparent to those skilledin the art without departing from the spirit and scope of the claimedinvention.

The use of headings and sections in the application is not meant tolimit the invention; each section can apply to any aspect, embodiment,or feature of the invention.

Throughout the application, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including or comprising specific process steps, itis contemplated that compositions of the present teachings also consistessentially of, or consist of, the recited components, and that theprocesses of the present teachings also consist essentially of, orconsist of, the recited process steps.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components and can be selected from a groupconsisting of two or more of the recited elements or components.Further, it should be understood that elements and/or features of acomposition, an apparatus, or a method described herein can be combinedin a variety of ways without departing from the spirit and scope of thepresent teachings, whether explicit or implicit herein.

The use of the terms “include,” “includes,” “including,” “have,” “has,”or “having” should be generally understood as open-ended andnon-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. Moreover, the singular forms “a,”“an,” and “the” include plural forms unless the context clearly dictatesotherwise. In addition, where the use of the term “about” is before aquantitative value, the present teachings also include the specificquantitative value itself, unless specifically stated otherwise.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the present teachings remainoperable. Moreover, two or more steps or actions may be conductedsimultaneously.

Where a range or list of values is provided, each intervening valuebetween the upper and lower limits of that range or list of values isindividually contemplated and is encompassed within the invention as ifeach value were specifically enumerated herein. In addition, smallerranges between and including the upper and lower limits of a given rangeare contemplated and encompassed within the invention. The listing ofexemplary values or ranges is not a disclaimer of other values or rangesbetween and including the upper and lower limits of a given range.

It is to be understood that the figures and descriptions of theinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the invention, while eliminating, forpurposes of clarity, other elements. Those of ordinary skill in the artwill recognize, however, that these and other elements may be desirable.However, because such elements are well known in the art, and becausethey do not facilitate a better understanding of the invention, adiscussion of such elements is not provided herein. It should beappreciated that the figures are presented for illustrative purposes andnot as construction drawings. Omitted details and modifications oralternative embodiments are within the purview of persons of ordinaryskill in the art.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

What is claimed is: 1.-23. (canceled)
 24. An intravascular imagingsystem comprising: a sheath defining a lumen, a first end and a secondend, the sheath having a first acoustic attenuation; a marker comprisingan optically opaque material, wherein the optically opaque material hasa second acoustic attenuation, wherein the marker is positioned betweenthe first end of the sheath and the second end of the sheath; and aprobe tip disposed within the sheath, the probe tip comprising anoptical sensor and an acoustic sensor.
 25. The system of claim 24further comprising an optical fiber segment disposed in the sheath,wherein the optical sensor is a beam director, wherein the optical fibersegment is in optical communication with the beam director.
 26. Thesystem of claim 24 wherein the first acoustic attenuation and the secondacoustic attenuation differ.
 27. The system of claim 24 wherein themarker is a disposed on or in the sheath.
 28. The system of claim 24wherein the marker is a cylindrical annulus.
 29. The system of claim 24wherein the marker comprises a marker surface, wherein the markersurface defines a discontinuity.
 30. The system of claim 28 wherein thecylindrical annulus defines a gap.
 31. The system of claim 30 whereinthe optical sensor and an acoustic sensor are arranged to rotate withinthe sheath relative to the gap such that optical signals and acousticsignals from are transmitted through the gap to each respective sensor.32. The system of claim 24 wherein the marker is a bar positioned on orwithin the sheath.
 33. The system of claim 24 further comprising ananalysis and instrumentation unit comprising a processor, wherein theprocessor is configured to locate the marker in a plurality ofintravascular images and register two or more of the plurality ofintravascular images by rotating the two or more images.
 34. The systemof claim 24 further comprising an analysis and instrumentation unitcomprising a processor, wherein the processor is configured to locatethe marker in a plurality of intravascular images and determine an angleof rotation from the marker location.
 35. The system of claim 34 whereinthe angle of rotation results in two or more intravascular images beingin register when one such image is rotated relative to the other suchimage by the angle of rotation.
 36. The system of claim 33 furthercomprising a display in communication with the analysis andinstrumentation unit, wherein the processor is configured to displayintravascular ultrasound images and intravascular optical imagesregistered using the marker.
 37. The system of claim 24 wherein theoptical sensor is an optical coherence tomography sensor.
 38. The systemof claim 24 further comprising a processor, wherein the marker definesan asymmetry, wherein the processor is configured to detect theasymmetry in a first image generated using data from the optical sensorand in a second image generated using data from the acoustic sensor. 39.The system of claim 38 wherein the processor is further configured toregister the first image and the second image using the asymmetry. 40.The system of claim 24 wherein the acoustic sensor is an ultrasoundtransducer.